The present invention relates generally to an apparatus for the electronically controlled, electromagnetic three-position selection of needles for a circular knitting machine with alternate motion, and in particular to circular knitting machine for the manufacturing of socks with inlay patterns.
It is known that in circular knitting machines, selection groups are utilized which are disposed around the needle cylinder, before each feeder or fall. Each selection group is made up of an electromagnet interposed between two permanent magnets, and of a pressure cam located in correspondence with the magnet disposed upstream of the electromagnet in the feeding direction of the cylinder, in order to push, against the latter, the oscillating selector associated with a spring pusher jack.
The electromagnet is driven--according to an electronic program synchronized with the motion of the needles cylinder--through a modulated passage. An interruption or a reversal of the current direction is provided for selectively keeping the oscillating selector attracted and adherent to the magnets, preventing the heel of three elastic jack from engaging a driving cam and for causing the disengagement of the corresponding needle. This arrangement is also provided for releasing (driving back) the oscillating selector thereby causing the lifting of the heel of the elastic jack by the same driving cam and the engagement of the corresponding needle.
Accordingly, for each feeder, as many selection groups are needed as the driving cam. Where it is desirable to have three-position section of the needles (that is, for floated stitch, retained stitch and discharged stitch) two selection groups are needed for each feeder and, in case of a circular machine with alternate motion, four selection groups are needed as disclosed in U.S. Pat. No. 3,972,206 and in the Italian application Ser. No. 9472-A/84.
By using the so-called "memory" systems, of the electromagnetical type for example, it is possible to reach a solution with three selection groups in which the central group cooperates alternatively with either of the side groups, but, as far as the machines for small-diameter socks are concerned, some dimensional, reliability and mechanical problems arise which make this solution unacceptable. By using an electromagnetic selection, one encounters the problem of the pressure cams, which are provided for pressing the oscillating selector against the magnet, being upstream of the selection electromagnet on every selection and in both directions of rotation. These cams, when of a fixed type, are incompatible with the needle path or otherwise, require complicated synchro devices.
Moreover, the electromagnet must exhibit a maximum width equal to the distance between one needle and the next in order to act one at a time on the single selector without interfering with adjacent ones.
The major problems concerning the known electromagnetic systems are therefore related to the available space and to the attraction force provided by the selection electromagnets. In fact, the space required to select three positions of the needles in both directions of the needles cylinder motion in a four-feeder machine for manufacturing socks with inlay patterns, for example, is such as to not allow the utilization of cylinders of a diameter smaller than four inches, while making it necessary to provide a cylinder diameter of 33/4" or even of 31/2" when a fineness exceeding 120 needles is required. Besides, the attraction or repulsion force of the selection electromagnet, being dependent on the cross-section of the magnetic flux passageway, is directly dependent on the needles pitch as mentioned above. Accordingly, in case of great fitness, the attraction or repulsion force is much reduced and, once the saturation level has been reached, it cannot be increased any longer even if the number of the coils or the current intensity is increased. The weakness of the magnetic force makes the balance among the elastic reaction of the selector, the inertia of the moving parts, centrifugal force, friction, etc., very uncertain thereby disadvantageously limiting the maximum available speed and the operation safety. To reduce the wear of the magnets, the surface which the magnet-attracted selector members are to slide on has been made of non-magnetic material, but this exerts a tangential force which pushes the selector members against the slot walls thereby increasing the friction forces and hindering the free movement of the same members proportionally to the operation speed. Finally, in order to obtain the elastic reaction of the selector being loaded at one end, a joint pressure is to be maintained at the other end which gives rise to an undesirably high sliding force when considering the length of the selector.